Process and plant for producing liquefied natural gas

ABSTRACT

A process for producing liquefied natural gas, in which natural gas feed having methane and higher hydrocarbons including benzene is cooled down to a first temperature level in a first cooling step using a first mixed coolant and then subjected to a countercurrent absorption using an absorption liquid to form a methane-enriched and benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed coolant and liquefied to give the liquefied natural gas. In the plant proposed, the first and second mixed coolants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping. The present invention likewise provides a corresponding plant.

The invention relates to a process and to a plant for producing benzene according to the preambles of the independent claims.

PRIOR ART

For liquefaction and non-pressurized storage, natural gas must be cooled down to low temperatures of approximately −160° C. In this state, the liquefied natural gas can be economically transported by cargo ship or truck, since it has only 1/600th of the volume of the gaseous substance at atmospheric pressure.

Natural gas generally contains a mixture of methane and higher hydrocarbons, along with nitrogen, carbon dioxide, and further undesirable constituents. Prior to liquefaction, these components must be partially removed in order to avoid solidification during liquefaction or in order to satisfy customer requirements. The methods used for this purpose, such as adsorption, absorption and cryogenic rectification, are generally known.

For details of methods used in natural gas liquefaction, reference is made to technical literature, such as the article “Natural Gas” in Ullmann's Encyclopedia of Industrial Chemistry, online publication Jul. 15, 2006, DOI: 10.1002/14356007.a17_073.pub2, in particular Section 3, “Liquefaction.”

In particular, mixed refrigerants consisting of various hydrocarbon components and nitrogen are used in natural gas condensing processes. For example, methods in which two mixed refrigerant circuits are used (dual mixed refrigerant (DMR) are known. In this way, natural gas, for example, which, in addition to methane, contains higher hydrocarbons, such as ethane, propane, butane, etc., but has already been freed of acid gases and dried beforehand, can be subjected to separation of the higher hydrocarbons and subsequent liquefaction. The separation of the higher hydrocarbons is accompanied by a separation of benzene, which is undesirable in the remaining liquefied natural gas. Benzene is used as a key or marker component in corresponding methods and can also be used as an indicator component for the separation.

Methods known from the prior art for natural gas liquefaction using corresponding mixed refrigerant circuits are often proven to be in need of improvement in practice for the reasons explained below.

The object of the present invention is, therefore, to improve natural gas liquefaction using two mixed refrigerant circuits.

DISCLOSURE OF THE INVENTION

Against this background, the present invention proposes a process for producing liquefied natural gas and a corresponding plant according to the preambles of the respective independent claims. Each of the embodiments are the subject matter of the dependent claims and of the description below.

Prior to explaining the features and advantages of the present invention, some of the principles of the present invention are explained in greater detail and terms used below are defined.

The present application uses the terms “pressure level” and “temperature level” to characterize pressures and temperatures, which is supposed to mean that corresponding pressures and temperatures in a corresponding plant do not have to be used in the form of exact pressure or temperature values. However, such pressures and temperatures typically fall within certain ranges that are, for example, ±10% about an average. In this case, corresponding pressure levels and temperature levels can be in disjointed ranges or in ranges which overlap one another. In particular, pressure levels, for example, include unavoidable or expected pressure losses. The same applies to temperature levels. The pressure levels indicated here in bar are absolute pressures.

Where “expansion machines” are referred to here, they are typically understood to mean known turboexpanders, which have radial impellers arranged on a shaft. A corresponding expansion machine can, for example, be mechanically braked or coupled to a device, such as a compressor or a generator. Expansion of a mixed refrigerant within the scope of the present invention is typically carried out using an expansion valve and not using an expansion machine.

A “heat exchanger” for use in the context of the present invention can be designed in any manner constituting usual practice in the field. It serves for the indirect transfer of heat between at least two fluid flows guided, for example, in countercurrent, here in particular a comparatively warm natural gas feed flow or a gaseous fraction formed therefrom and one or more cold mixed refrigerant flows. A corresponding heat exchanger can be formed from one or more heat exchanger sections connected in parallel and/or in series, e.g., from one or more wound heat exchangers or corresponding sections. In addition to wound heat exchangers of the type already mentioned, other types of heat exchangers may also be used within the scope of the present invention.

The relative spatial terms “upper,” “lower,” “over,” “under,” “above,” “below,” “adjacent to,” “next to,” “vertical,” “horizontal,” etc. here refer to the reciprocal arrangement of components during normal operation. An arrangement of two components “one above the other” is understood here to mean that the upper end of the lower of the two components is located at a lower geodetic height than the lower end of the upper of the two components or at the same geodetic height thereas, and the vertical projections of the two components overlap. In particular, the two components are arranged exactly one above the other, that is to say the central axes of the two components run on the same vertical straight line. However, the axes of the two components need not lie exactly vertically one above the other, they may also be offset from one another.

In the context of the present invention, a countercurrent absorber is used. As regards the design and configuration of corresponding apparatuses, reference is made to relevant textbooks (see, for example, K. Sattler: Thermische Trennverfahren. Grundlagen, Auslegung, Apparate. Weinheim: Wiley-VCH, 3rd Edition 2001). A liquid fraction (“sump liquid”) and a gaseous fraction (“head gas”) can typically be removed from a lower region (“sump”) or from an upper region (“head”). Countercurrent absorbers are also generally known from the field of separation technology. They are used for absorption in the phase countercurrent and are therefore also referred to as countercurrent columns. During absorption in the countercurrent, the releasing gas phase flows upwards through an absorption column. The receiving solution phase, provided from above and withdrawn at the bottom, flows towards the gas phase. The gas phase is “washed” with the solution phase. Built-in components that ensure a gradual (trays, spray zones, rotating plates, etc.) or continuous (random filling of filling material, packings, etc.) phase contact are typically provided in a corresponding absorption column. A liquid stream, also referred to as “absorption liquid,” is fed into an upper region of a countercurrent absorber, whereby components are washed out of a gaseous stream that is fed in more deeply.

Where a “feed of natural gas” is referred to below, this is to be understood to mean natural gas that has been subjected, in particular, to acid gas removal and optional further conditioning. In particular, heavy hydrocarbons, such as butanes and/or pentanes, along with hydrocarbons having six or more carbon atoms, may already have been separated from corresponding feed of natural gas. The feed of natural gas is, in particular, anhydrous and has a content of, for example, more than 85% methane and contains, in particular, ethane and propane in the remainder. Nitrogen, helium and other light components may also still be contained.

Where “liquefied natural gas” is referred to below, it is understood to mean a cryogenic liquid at the atmospheric boiling point of methane or below, especially at −160 to −164° C., which comprises more than 85% methane, especially more than 90% methane, and the methane content of which is in any case higher than that of the natural gas used. The liquefied natural gas is, in particular, significantly lower in benzene than the feed of natural gas and comprises benzene only at a maximum content given below.

FEATURES AND ADVANTAGES OF THE INVENTION

A method for producing liquefied natural gas using two mixed refrigerants is disclosed, for example, in U.S. Pat. No. 6,119,479. In this process, the higher hydrocarbons contained in the natural gas feed can be separated from it this in a countercurrent absorber as needed.

For this purpose, the natural gas feed can be cooled down in a first colling step, depending on the composition, to a temperature in the range of −20° C. to −70° C. and then fed into the countercurrent absorber. The countercurrent absorber can have a sump heater. Sump liquid separated in the countercurrent absorber contains at least a portion of the higher hydrocarbons from the natural gas feed. The sump liquid can be returned to the countercurrent absorber in part as absorption liquid and, if necessary, also partially supplied to a head gas of the countercurrent absorber after its removal from the countercurrent absorber. In this way, the head gas of the countercurrent absorber is depleted of at least a portion of the higher hydrocarbons and is subsequently subjected to a second cooling step, which initiates the liquefaction. Benzene is also used as the key component here, which benzene may be contained in the head gas of the countercurrent absorber, and thus in the natural gas to be liquefied, in particular at less than 1 ppm on a molar basis. The contents of other higher hydrocarbons result therefrom; however, these are typically less critical. Benzene is, in particular, to be regarded as critical in natural gas liquefaction because it can solidify at the low temperatures used.

Mixed refrigerants are used in corresponding refrigerant circuits both in the first and in the second cooling step of the method just explained. In particular, a first mixed refrigerant (warm mixed refrigerant, WMR) is subjected to compression in gaseous form in the order indicated below, condensed by cooling, subcooled, expanded, heated in the first heat exchanger, in particular completely evaporated thereby, and subsequently subjected to compression again. The first mixed refrigerant can be subcooled in particular in the first heat exchanger, the previous cooling takes place in a further heat exchanger. Furthermore, a second mixed refrigerant (cold mixed refrigerant, CMR) can be subjected to compression in gaseous form, condensed subcooled by cooling, relaxed, heated in the second heat exchanger, in particular completely evaporated thereby, and subsequently subjected to compression again. The subcooling of the second mixed refrigerant can take place in particular in the second heat exchanger, the previous cooling in the first and the second heat exchangers.

The first and second heat exchangers are in particular used in a known manner per se as coiled heat exchangers (coil wound heat exchanger, CWHE), wherein the heating of the mixed refrigerant takes place, after its expansion, in particular on the shell side, i.e., in a jacket space surrounding the heat exchanger tubes or, into which the mixed refrigerant is expanded. The media to be cooled down are guided tube-side, i.e., through the correspondingly provided heat exchanger tubes. The heat exchanger tubes are provided in bundles in corresponding heat exchangers, so that the term “tube-side” or “bundle-side” is used here for a corresponding flow guide.

Processes and plants of a similar type are also disclosed, for example, in U.S. Pat. No. 6,370,910 A and AU 2005224308 B2.

Processes for natural gas liquefaction must be able to be flexibly adapted to different plant capacities and operating conditions. The processes as explained, which use two mixed refrigerant circuits, are preferably used, where large ambient temperature fluctuations result in significantly different refrigerant condensation conditions. These can be taken into account more efficiently if a mixture comprising refrigerant components is used instead of a single pure component, such as propane.

In addition, corresponding processes do not contain large inventories of liquid hydrocarbons having a higher molecular weight than air, which would present a significant safety risk. Corresponding hydrocarbons can accumulate in more deeply located regions and possibly lead to explosions. Propane in this sense is considered to be the most dangerous refrigerant due to a combination of high volatility and high molecular weight. Processes using two mixed refrigerant circuits and a correspondingly reduced propane fraction therein are, therefore, a preferred solution for plant layouts having limited installation space, for example modularized plants and/or floating plants, in which the base area is limited.

A compact plant layout (e.g., mandatory for offshore installations) can be achieved by minimizing the number of plant components and by reducing the space between the plants, which can be determined based on safety aspects. The plant components known to be hazardous include pumps for liquid hydrocarbons (risk of leakage and liquid discharge) and all types of devices that contain significant amounts of liquid propane.

The present invention solves the problems explained by dispensing with hydrocarbon pumps and largely dispensing with propane as a refrigerant component in corresponding mixed refrigerants. These advantages are achieved by the measures according to the invention proposed below and corresponding advantageous embodiments.

In the process proposed according to the invention for producing liquefied natural gas, a natural gas feed of the type explained above, which contains methane and higher hydrocarbons, including benzene, is cooled down as a whole in a first cooling step using a first (“warm”) mixed refrigerant cooled down to a first temperature level, in particular from −20° C. to −70° C., and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction. The benzene-depleted gas fraction has, in particular, a content of less than 1 ppm on a molar basis of benzene, wherein the content of benzene in the natural gas feed is significantly higher, for example 5 to 500 ppm. Particularly in comparison to the natural gas feed, the gas fraction formed is enriched in methane and depleted of the higher hydrocarbons.

Known means can, in principle, be used for countercurrent absorption. The gas fraction can also be (essentially) free of hydrocarbons having five and optionally more carbon atoms, so that depletion (essentially) to zero can take place. However, higher hydrocarbons may also still be contained, and a sump liquid formed during countercurrent absorption can also have certain proportions of methane. The degree of separation or accumulation and depletion achieved in countercurrent absorption depends upon the subsequent use of corresponding fractions and the respective tolerable contents of the specified components.

In the context of the present invention, a portion of the gas fraction from countercurrent absorption, which is correspondingly depleted (or essentially free) of benzene (and other higher hydrocarbons), is cooled down in a second cooling step using a second (“cold”) mixed refrigerant cooled down to a second temperature level of, in particular, −145° C. to −165° C. and liquefied to give liquefied natural gas. Liquefied natural gas formed in this way can be subjected to any further processing or conditioning (expansion, subcooling, etc.).

In the context of the present invention, the first and second mixed refrigerants are low in propane (having a content of less than 5 mole percent propane) or (essentially) propane-free, and the absorption liquid for the countercurrent absorption is formed from a further portion of the gas fraction from the countercurrent absorption, which (geodetically) is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping. For the term “above,” reference is made to the above definitions.

By means of the proposed measures, the present invention reduces or eliminates the use of appreciable amounts of propane-containing media. As mentioned, propane is considered a dangerous refrigerant due to the combination of high volatility and high molecular weight. A corresponding refrigerant must necessarily be conveyed by means of machines, with which there is an increased probability of propane leakage. This is no longer the case within the scope of the present invention, which means that it is also suitable and advantageous, in particular, for plant layouts having limited installation space, for example modularized plants and/or floating plants with which the base area is limited and additional installation space requiring safety equipment can only be installed with difficulty.

Since the absorption liquid for countercurrent absorption is formed from the further portion of the gas fraction from the countercurrent absorption, condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping, this (possibly propane-containing) medium does not require the detrimental use of pumps having the problems explained.

The invention thus provides a solution in which the use of significant amounts of propane-containing media is essentially dispensed with, either by using refrigerant mixtures that previously contained propane in a low-propane or propane-free manner or by conveying a propane-containing head gas from the countercurrent absorption without pumping. Surprisingly, it has been found out that the process proposed in the context of the present invention has the same or a higher thermodynamic efficiency in comparison to known methods. Within the scope of the present invention, the investment costs can be reduced without increasing the operating costs.

In the process proposed according to the invention, a countercurrent absorber is advantageously used in countercurrent absorption, which countercurrent absorber is operated with a head condenser arranged above an absorption region of the countercurrent absorber, wherein the head condenser is used to condense the further portion of the gas fraction. An “absorption region” is to be understood here as meaning the region having built-in components as explained above.

The head condenser can be integrated into the countercurrent absorber or at least partially arranged within the countercurrent absorber. An integrated head condenser comprises a heat exchange structure in a common column jacket, in which material exchange structures of the type explained above are also arranged, wherein the heat exchange structure, for example, a cooling coil or the like is separated from a region containing the material exchange structures, in particular by a liquid accumulation bottom or a liquid-tight bottom.

The latter allows a controlled return of condensate to the region having the material exchange structures. In contrast, a head condenser arranged outside is not arranged in a common column jacket having the material exchange structures.

In the process proposed according to the invention, the first refrigerant mixture advantageously comprises in total more than 90 mole percent ethane, isobutane and n-butane and in total less than 10, preferably less than 5 mole percent nitrogen, methane, propane and hydrocarbons having five or more carbon atoms. Compared to known processes, the small amount of propane proves unproblematic. By contrast, the second mixed refrigerant advantageously has more than 98 mole percent nitrogen, methane and ethane in total and less than 2 mole percent propane and higher hydrocarbons in total.

In the context of the present invention, a first heat exchanger is advantageously used in the first cooling step, wherein the first mixed refrigerant in gaseous form is subjected to, in particular, single-stage compression in a first mixed refrigerant circuit, condensed by cooling, subcooled, relaxed, heated in the first heat exchanger, in particular completely evaporated thereby, and subsequently subjected to compression again. The subcooling of the first mixed refrigerant can take place, in particular, in the first heat exchanger, the previous cooling in a further heat exchanger. In contrast to processes not according to the invention, the compression of the first mixed refrigerant thus takes place, in particular, in a single stage and without intermediate cooling, which would constitute a risk of partial condensation and a need to convey the condensate to the high-pressure side of the compressor. This disadvantage is remedied here.

Furthermore, in the method according to the invention, a second heat exchanger is advantageously used in the second cooling step, wherein the second mixed refrigerant in a second mixed refrigerant circuit is subjected to, in particular multi-stage, compression, condensed by cooling, subcooled, relaxed, heated in the second heat exchanger, in particular completely evaporated thereby, and subsequently subjected to compression again. The subcooling of the second mixed refrigerant can take place, in particular, in the second heat exchanger, the previous cooling in the first and the second heat exchangers.

As mentioned, the first and second heat exchangers may be designed as wound heat exchangers and, in particular, with one or two (serial) bundles in a common jacket in each case.

In the context of the present invention, a header for the second mixed refrigerant, which receives said refrigerant after it has condensed, can be designed in particular for a pressure that is 2 to 10 bar above a suction pressure of a compressor or of a first of several compressors used in compressing the second mixed refrigerant.

In particular, a series of three compressors can be used for compressing the first and the second mixed refrigerant, a first of which compresses the first mixed refrigerant and the further two compress the second mixed refrigerant. Such compressors can be designed for (almost) identical shaft powers, i.e., 33⅓±3% of the total power consumption.

The second mixed refrigerant is advantageously used after heating and evaporation in the second heat exchanger and before compression during the condensation of the further portion of the gas fraction from the countercurrent absorption and is further heated thereby. In this way, a particularly advantageous utilization of this second mixed refrigerant results.

In the context of the present invention, the first (but not the second) heat exchanger is advantageously used for cooling the first mixed refrigerant and/or the second (and additionally the first) heat exchanger is used for cooling the second mixed refrigerant. Further cooling after compression or after compression steps can take place in a known manner, for example using air or water coolers.

In the context of the present invention, in an alternative, a rising gas phase is formed in countercurrent absorption at least in part by feeding in further natural gas feed, which was subjected to the first cooling step. In this way, there is no need for a reboiler, but a higher separation performance is caused in countercurrent absorption. However, the rising gas phase can also be provided at least in part by evaporation of a portion of a sump liquid formed in the countercurrent absorption.

In the context of the present invention, working liquid expanders can be used instead of expansion valves. This reduces energy consumption.

The present invention is suitable for typical natural gases so that the natural gas feed can contain, in particular, at least 80% methane and, in the methane-free remainder, at least 50% ethane and propane. The liquefied natural gas advantageously contains at least 90% methane, wherein a methane content in the liquefied natural gas is higher than in the natural gas feed.

The present invention further extends to a plant for producing benzene, reference being made to the corresponding independent claim with regard to its specific features. For further features and embodiments of such a plant and of preferred embodiments, reference is expressly made to the above explanations regarding the method according to the invention and its respective advantageous embodiments. Advantageously, such an arrangement is designed for carrying out a process as previously explained in different embodiments.

The invention is described in more detail hereafter with reference to the accompanying drawings, which illustrate a natural gas liquefaction plant according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a plant according to an embodiment of the present invention in the form of a simplified process flow diagram.

FIG. 2 illustrates a plant according to a further embodiment of the invention in the form of a simplified process flow diagram.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, a plant according to a particularly preferred embodiment of the present invention is shown in the form of a greatly simplified schematic process flow diagram and is designated as a whole by 100.

The plant 100 illustrated in FIG. 1 is supplied with natural gas feed NG, which is first divided into two partial streams. A first partial stream is cooled down in a first heat exchanger E1, which can in particular be designed as a wound heat exchanger, in a first cooling step to a first temperature level of, for example, −20° C. to −70° C. and then fed approximately centrally into a countercurrent absorber T1.

Furthermore, the second partial flow of the natural gas feed NG, which is expanded via a valve V6, is fed into a lower region of the countercurrent absorber T1, where it rises essentially in gaseous form. Gas is withdrawn from an upper region of the countercurrent absorber T1 and is cooled down in a head condenser E2, which can be designed, for example, as a plate heat exchanger, and fed into a head space of the countercurrent absorber T1. Liquid precipitating here is returned as a return flow to the countercurrent absorber T1 and washes out heavier components from the natural gas feed, which pass into a sump liquid of the countercurrent absorber T1.

The sump liquid of the countercurrent absorber T1 can be expanded via a valve V5 and discharged from the plant 100 as a heavy fraction HHC (heavy hydrocarbon). Head gas of the countercurrent absorber T1, i.e., a methane-rich gas fraction, is, in contrast, cooled down to a liquefaction temperature in a second heat exchanger E3, which can also be designed as a wound heat exchanger, and, after expansion, discharged via a valve V4 as liquefied natural gas LNG from the plant 100.

The plant 100 comprises two mixed refrigerant circuits. In a first mixed refrigerant circuit, a first (“warm”) mixed refrigerant WMR is subjected to single-stage compression in gaseous form in a compressor C1 and subsequently cooled down in an air cooler and/or water cooler E4 and thereby condensed. Condensate can be obtained in a separation vessel D1. This is first further cooled down in the first heat exchanger E1 on the bundle side, then expanded via a valve V1 and fed into the jacket space of the first heat exchanger E1, where it is heated, completely evaporated and subsequently subjected to compression again.

In contrast to processes not according to the invention, the compression of the first mixed refrigerant takes place here, in particular, in the single-stage compressor C1 without intermediate cooling, which would constitute a risk of partial condensation and a need to convey the condensate to the high-pressure side of the compressor. This disadvantage is remedied here.

Furthermore, in the plant 100, a second mixed refrigerant CMR is subjected to a gradual compression in compressors LP C2 and HP C2 in gaseous form and subsequently cooled down in each case, for example in air coolers and/or water coolers E5 and E6. Further cooling takes place on the bundle side in the first heat exchanger E1 and then in the second heat exchanger E3. After subsequent expansion in a valve V2, feeding into a buffer vessel D2 takes place. Condensate withdrawn therefrom is expanded via a valve V3 and fed jacket-side into the second heat exchanger E2, where it is heated and completely evaporated. The gaseous second mixed refrigerant CMR is used as refrigerant in the aforementioned head condenser E2 before it is again subjected to compression.

A return pump can be dispensed with by installing the head condenser E2, which is operated using tactile heat of the second mixed refrigerant, which leaves the second heat exchanger E3 as a vapor above the countercurrent absorber T1. In contrast, the return flow formed from the gas from the countercurrent absorber T1 is returned to the countercurrent absorber T1 purely by the effect of gravity.

In FIG. 2, a plant according to a further embodiment of the present invention is shown in the form of a greatly simplified schematic process flow diagram and is designated as a whole by 200.

A first difference from the embodiment of the plant 100 according to FIG. 1 here is that the countercurrent absorber T1 is not supplied with a partial flow of the natural gas feed, but instead a reboiler E7 is provided which evaporates a portion of the sump liquid of the countercurrent absorber T1 and thus forms a portion of the rising gas phase in the countercurrent absorber T1.

A further difference from the embodiment of the plant 100 according to FIG. 1 is, furthermore, that the head condenser E3 in the form of corresponding heat exchanger structures is displaced into the head space of the countercurrent absorber T1, whereby corresponding installation space is potentially saved.

Finally, as illustrated here, an expansion of the liquid natural gas LNG leaving the second heat exchanger E3 is provided via an expansion machine

X1 and a corresponding expansion of the cooled second mixed refrigerant CMR in an expansion machine X2. Analogously, the valve V1 can also be replaced by a expansion machine X3 (not shown). 

1-14. (canceled)
 15. A process for producing liquefied natural gas, in which natural gas feed containing methane and higher hydrocarbons, including benzene, is cooled down to a first temperature level in a first cooling step using a first mixed refrigerant, and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed refrigerant and liquefied to give the liquefied natural gas, wherein the first and second mixed refrigerants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping.
 16. The process according to claim 15, wherein a countercurrent absorber is used in the countercurrent absorption, which is operated with a head condenser arranged above an absorption region of the countercurrent absorber, wherein the head condenser is used for condensing the further portion of the gas fraction.
 17. The process according to claim 16, wherein the head condenser is integrated into the countercurrent absorber or is at least partially arranged within the countercurrent absorber.
 18. The process according to claim 15, wherein the first mixed refrigerant comprises in total more than 90 mole percent, preferably more than 95 mole percent ethane, isobutane and n-butane and in total less than 10 mole percent, preferably less than 5 mole percent nitrogen, methane, propane and hydrocarbons having five or more carbon atoms.
 19. The process according to claim 15, wherein the second mixed refrigerant comprises in total more than 98 mole percent nitrogen, methane and ethane and in total less than 2 mole percent propane and heavier hydrocarbons.
 20. The process according to claim 15, wherein a first heat exchanger is used in the first cooling step, wherein the first mixed refrigerant in gaseous form is subjected to, in particular, single-stage compression in a first mixed refrigerant circuit, condensed by cooling, subcooled, expanded, heated in the first heat exchanger and, in particular, completely evaporated thereby, and subsequently subjected to the compression again.
 21. The process according to claim 20, wherein a second heat exchanger is used in the second cooling step, wherein the second mixed refrigerant in gaseous from is subjected to an in particular multi-stage compression in a second mixed refrigerant circuit, condensed by cooling, subcooled, expanded, heated in the second heat exchanger and, in particular, completely evaporated thereby, and subsequently subjected to compression again.
 22. The process according to claim 21, wherein the second mixed refrigerant is used after heating in the second heat exchanger and before compression during the condensation of the further portion of the gas fraction from the countercurrent absorption and further heated thereby.
 23. The process according to claim 21, wherein the first heat exchanger (E1) is used to cool down the first mixed refrigerant (WMR) and/or the first (E1) and the second (E3) heat exchangers are used to cool down the second mixed refrigerant (CMR).
 24. The process according to claim 15, wherein in countercurrent absorption, a rising gas phase is provided at least in part by feeding in further natural gas feed which was not subjected to the first cooling step and/or at least in part by evaporating a portion of a sump liquid formed in the countercurrent absorption.
 25. The process according to claim 15, wherein the natural gas feed contains at least 80% methane and, in the methane-free remainder, at least 50% ethane and propane.
 26. A method according to claim 15, wherein the liquefied natural gas contains at least 90% methane, wherein the methane content in the liquefied natural gas is higher than in the natural gas feed.
 27. A plant configured to produce liquefied natural gas, having a first heat exchanger configured to cool natural gas feed containing methane and higher hydrocarbons, including benzene, to a first temperature level in a first cooling step using a first mixed refrigerant, a countercurrent absorber configured to subject the natural gas feed to countercurrent absorption using an absorption liquid after the first cooling step by forming a benzene-depleted gas fraction, having a second heat exchanger configured to cool down a portion of the gas fraction in a second cooling step to a second temperature level using a second mixed refrigerant and is liquefied to give the liquefied natural gas, wherein the plant is configured to use low-propane or propane-free first and second mixed refrigerants, and means are provided which are configured to form the absorption liquid from a further portion of the gas fraction, wherein they condense this above the countercurrent absorption and return it to the countercurrent absorption without pumping.
 28. The plant according to claim 27, wherein the plant is configured to carry out a process for producing liquefied natural gas, in which natural gas feed containing methane and higher hydrocarbons, including benzene, is cooled down to a first temperature level in a first cooling step using a first mixed refrigerant, and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed refrigerant and liquefied to give the liquefied natural gas, wherein the first and second mixed refrigerants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping. 